1. Field of the Invention
The present invention relates to an FSK demodulating device located at a receiving side in a data transmission system in which digital data is formatted in a transmission frame to transmit the digital data as a frequency shift keying (FSK) signal, for demodulating the received frequency shift keying (FSK) signal to an original digital data signal.
2. Description of the Related Art
A data transmission system in which digital data is formatted in a transmission frame to transmit the digital data as a frequency shift keying (FSK) signal has been practically used as a means for accurately transmitting the digital data through a data communication channel or a radio channel for a long distance. FIG. 1 is a block diagram showing e.g., a data transmission system for transmitting data obtained by a measuring instrument to a host device through a radio channel.
The data measured by measuring instrument 1 is converted into e.g., 8-bit digital data by data processing circuit 2. The 8-bit digital data is formatted in a serial transmission frame and applied to voltage-controlled oscillator (VCXO) 3 as digital data signal a. Digital data signal a is modulated to frequency shift keying (FSK) signal b by voltage-controlled oscillator 3. Modulated signal b is amplified by amplifier 4 and output through a radio channel via antenna 5.
Frequency shift keying signal b output through the radio channel from antenna 5 is received by antenna 6 at a receiving side. Received signal b is amplified by amplifier 7 and unnecessary frequency components thereof are removed. Then, signal b is input to FSK demodulating device 8. Frequency shift keying signal b input to FSK demodulating device 8 is converted into voltage signal c by detector 9 and discriminated to the original digital data signal d by discriminator 10.
Voltage-controlled oscillator 3 described above serves as e.g., an oscillator in which quartz is used as an oscillating element. As shown in FIG. 2, oscillation frequency f of oscillator 3 is changed in proportion to input voltage value V. As shown in FIG. 3A, assume that when digital data signal a is at H level, its signal value is e.g., 2.0 V, and when signal a is at L level, its signal value is e.g., 1.5 V. When digital data signal a is kept at H level (V=VH), signal a serves as an output signal having frequency fH (=273.270 MHz). When digital data signal a is kept at L level, signal a serves as an output signal having frequency fL (=273.252 MHz). More specifically, frequency shift keying (FSK) signal b output from voltage-controlled oscillator 3 serves a combination signal consisting of frequencies fH and fL, which are respectively output in response to the H and L signal levels.
Detector 9 at a receiving side converts frequency shift keying signal b received by receiver 7 into voltage values VH (=2.0 V) and VL (=1.5 V) in correspondence with frequencies fH and fL of signal b and outputs voltage signal c, as shown in FIG. 3C.
As shown in FIG. 4, discriminator 10 includes voltage comparator 10a and pull-up resistor 10c. In voltage comparator 10a, voltage signal c is input to a (+) input terminal thereof, and e.g., reference voltage VR of 1.75 V from battery 10b is applied to a (-) terminal thereof. Pull-up resistor 10c is arranged to apply a control voltage of +5 V to an output terminal of voltage comparator 10a. Therefore, input voltage signal c is compared with reference voltage VR (=1.75 V), and digital data signal d (Vh=5 V, and Vl=0 V) is output from the output terminal, as shown in FIG. 3D.
The FSK demodulating device with the above arrangement, however, has the following problems. More specifically, a conventional FSK signal has two transmission methods. One of them is called subcarrier FSK or audio FSK. In this method, FM modulation of binary signal is performed with two audio frequencies. The other method is called direct FSK. In this method, as shown in FIGS. 1 and 2, a carrier itself is shifted to modulate the binary signal.
The direct FSK has the following advantages as compared to the subcarrier FSK or the audio FSK. That is, in the direct FSK, a higher transmission speed and a simple modulating device can be realized at low manufacturing cost. However, the direct FSK has the following disadvantage in its demodulator. More specifically, according to the conventional method a signal modulated with the direct FSK is demodulated to the voltage corresponding to the binary value through an FM demodulating device. At this time, if the frequency of the carrier is changed in accordance with the change in factors such as temperature, voltage, and the like, which affect the frequency, the voltage which is FM-demodulated is also changed. Therefore, the voltage comparator cannot accurately compare the voltages. The above arrangement will be described below in detail with reference to the accompanying circuit diagrams.
Voltage-controlled oscillator 3 at a transmitting side as described above has a quartz oscillating element. Therefore, as shown in FIG. 2, output frequency change .DELTA.f (=fH-fL) with respect to input voltage change .DELTA.V (=VH-VL) is hardly changed and a predetermined value is constantly maintained. For example, .DELTA.f=15 kHz at .DELTA.V=0.5 V, according to the arrangement in FIG. 2.
If an ambient temperature is changed, however, entire voltage frequency characteristics are shifted, as represented by a dotted line in FIG. 2. In other words, the characteristics are zero-point shifted in an axis direction of frequency f. Thus, if the voltage frequency characteristics are zero-point shifted, frequency values fH and fL of frequency shift keying signal b are equally increased from the corresponding reference values (273.270 MHz, 273.255 MHz).
Detector 9 in FSK demodulating device 8 at the receiving side, which has received frequency shift keying signal b including above frequencies fH and fL, detects frequencies fH and fL which are equally increased. Therefore, as shown in FIG. 5, voltages VH and VL of voltage signal b' output from detector 9 have signal waveforms zero-point shifted upward by dV as compared to voltages VH and VL of voltage signal b in a normal mode.
When voltage signal b' zero-point shifted by dV is applied to the (+) terminal of voltage comparator 10a in discriminator 10, since the value of reference voltage VR applied to the (-) terminal is not changed, reference voltage VR and voltage VL of voltage signal b' become similar to each other. Therefore, if some noise or the like is mixed in voltage signal b', discriminator 10 may be erroneously operated. In addition, when zero-point shift amount dV described above becomes larger than reference voltage VR, this discriminator 10 cannot undesirably discriminate voltage signal b'0 to the original data signal d.
It should be noted that even if the above-described voltage frequency characteristics are zero-point shifted in a (-) direction, or digital data signal a input to voltage-controlled oscillator 3 is largely zero-point shifted, oscillation frequencies fH and fL of voltage-controlled oscillator 3 are shifted to pose the above problems.